Near field communication (NFC) enabled devices such as mobile phones can establish communication with another device by touching the other device or being moved into close proximity with the other device. The other device can be as another mobile device, an NFC reader, such as a payment kiosk, or an NFC tag. NFC enabled devices have to be present within a relatively small distance from one another to allow information exchange through electromagnetic induction between their corresponding loop antennas. Ranges of up to several centimeters (e.g., up to about 10 cm) are common for many NFC devices. A first NFC device may transmit a magnetic field modulated with the information to be exchanged, such as credit card information for payment in a contactless financial transaction, or ticket fare information in an electronic ticketing transaction. A second NFC device nearby may receive the information via inductive coupling, and may respond to the first NFC device by transmitting or generating its own modulated magnetic field and inductively coupling this magnetic field to the first NFC device.
In another mode of operation, an NFC-enabled device may operate as an NFC reader and/or writer and communicate with an NFC tag, which is a passive data store that can be read, and under certain conditions, written to by an NFC device. NFC tags have no power source (e.g., battery) and can be custom-encoded by the manufactures or be encoded using industry specifications. An NFC reader can transmit a carrier signal (e.g., at 13.56 MHz) during reception. The carrier signal can provide energy to power the NFC tag. The NFC tag transmits data to the NFC reader by modulating the carrier signal with the data. The receiver circuit of the NFC reader has to demodulate the NFC tag response in the presence of its own transmit (TX) carrier signal.